Proper equipment and techniques that meet the challenge of finishing stainless steel welds
May 13, 2008
Stainless steel products are used in all parts of our daily life. Certain surface finish expectations must be met, and one of the major challenges in stainless steel treatment is meeting these expectations. The biggest problem has been the time required to rebuild the surface from a weld. While surface finishing of stainless steel still requires a good deal of effort, the proper grinding and polishing equipment makes it possible to create perfect finishes in a reasonable amount of time.
Stainless steel products are used in all parts of our daily life. Think about how many stainless steel surfaces your fingers touch before you even arrive at work. With the popularity of stainless steel has come the demand for high-quality stainless steel finishes.
Customers have certain expectations for the surface finish, and one of the major challenges in stainless steel treatment is meeting these expectations. The biggest problem has been the labor and time required to rebuild the surface from a weld (with all of its oxidations) to a perfect hairline or mirror finish. However, new technologies have made this process more efficient and less costly.
When a hairline finish must be restored, use an elastic flap disk to remove the weld. One type of abrasive disk that does this is made from a compound developed specifically for grinding stainless steel. The fiber in the compound is derived from the hemp plant, which provides excellent abrasion while also being environmentally friendly.
Heat absorbency is another characteristic of this material. When this type of abrasive flap disk is used on a variable-speed angle grinder at a maximum of 8,000 RPM, the grinding temperature will be reduced, helping to prevent discoloring. Furthermore, the service life of the disk will be increased.
After pregrinding with the angle grinder, you can remove scratches easily with a linear grinder, and build up a hairline finish. When choosing a linear grinder, look for one designed to accept hollow-core, slide-on abrasives, as this will save time and reduce your costs. Remember to grind in an oscillating (back and forth) manner. If grinding is done only lengthwise, the surface will be spoiled by new lines and streaks.
In the past mirror finishing of stainless steel required many steps and a lot of time. Even then a real mirror finish was impossible to achieve, and a high-gloss finish was the best that could be obtained. However, two recently developed abrasives have changed the whole process. An abrasive cloth with the grit arranged in a pyramid structure provides aggressive grinding, yet produces a very fine surface finish on stainless steel. It should be used to do pregrinding in two or three steps, ending with 1,200 grit.
The next step is to polish the surface with a gentler abrasive used in combination with polishing paste or cream. An absorbent fleece material is preferable to common polishing felt, which gets clogged with the polishing paste. Fleece eliminates smearing and results in an absolute mirror finish.
Both the abrasive cloth and the absorbent fleece are available as disks, sleeves, and belts, so that they can be used on flat surfaces with linear grinders, and on pipes with speed-regulated angle grinders. The most important thing to remember is to perform each grinding step lengthwise, across, and then lengthwise again while moving the machine in an oscillating manner.
You can take several steps to maximize finishing efficiency. First, let the weight of the machine apply the pressure to the workpiece, as this will help to reduce the grinding temperature. Also, be sure to keep your work area clean. One abrasive grain left over from pregrinding is enough to spoil the whole surface of your workpiece.
Clean the surface between every grinding step with a microfiber cloth and a cleaning agent made specifically for this purpose. It is best not to mix different polishing pastes on one absorbent fleece disk, sleeve, or belt.
If you use preground or prepolished stainless steel sheets for your fabrication, it is sometimes enough to rework the area of the weld without grinding and polishing the entire surface.
Highly compressed fleece disks handle difficult corner welds.
Corner welds can be very awkward to finish. Highly compressed fleece disks, available in a variety of grits, are made specifically for fillet and corner welds and provide a high level of abrasion and a long service life. (See Figure 1). You first should use 80 grit to remove scratches that occur from pregrinding to give the surface a nice hairline finish.
If you have a gas tungsten arc weld (GTAW), for example, 80 grit will be enough to grind down the weld and finish the surface in one step. Next, dry polishing with 900 grit will result in a mirror finish without polishing additives. Both grits can be used on any variable-speed angle grinder. However, you can use them only frontally and not on the flat side.
As a welding professional, you take great pride in the quality of your welds. By following a few simple suggestions, you can take as much pride in the quality of the final finish of each weld area. It is obvious that surface finishing of stainless steel still requires a good deal of effort, but with the proper grinding and polishing equipment, it is possible to create perfect finishes in a reasonable amount of time.